The present invention relates to a switching device for driving of a display panel which uses an organic EL, a liquid crystal or the like, or a switching device to be utilized in a high density memory or the like.
In recent years, the characteristics of an organic electronic material have remarkably been improved. Particularly, it has been considered as to whether a so-called organic bistable material in which, when a voltage is continuously applied on the material in an increasing manner, a current in a circuit is abruptly increased at a given voltage or more and a switching phenomenon is observed can be applied to a switching device for driving an organic EL display panel, a high density memory, or the like.
In FIG. 11, an example of voltage-current characteristics of an organic bistable material which shows such switching behavior as described above is shown. As shown in FIG. 11, the organic bistable material has two types of current-voltage characteristics, that is, high resistance characteristics 51 (off state) and low resistance characteristics 52 (on state) and also has non-linear response characteristics such that, in a state in which a bias of Vb is previously applied, when a voltage is allowed to be Vth2 or more, resistance is changed and, then, the off state is converted into the on state, whereas, when a voltage is allowed to be Vth1 or less, resistance is changed and, then, the on state is converted into the off state. Namely, by applying a voltage of Vth2 or more or Vth1 or less on the organic bistable material, a so-called switching behavior can be performed. On this occasion, Vth1 and Vth2 can be applied as a voltage in a pulse state.
As for organic bistable materials showing such non-linear response as described above, various types of organic complexes are known. For example, R. S. Potember et al. Appl. Phys. Lett. 34, (1979) 405 discusses experimentally manufacturing a switching device having two types of stable resistance against a voltage by using a Cu-TCNQ (copper-tetracyanoquinodimethane) complex. Further, Kumai et al. Solid Physics 35, (2000) 35 observes a switching behavior by a non-linear response using a monocrystal of K-TCNQ (potassium-tetracyanoquinodimethane) complex. Still further, Oyamada et al. Appl. Phys. Lett. 83, (2003) 1252 discusses forming a Cu-TCNQ complex thin film by using a vacuum evaporation method and, then, while clarifying switching characteristics thereof, possibility of application thereof to an organic EL matrix is studied and, on this occasion, formation of an alumina film between a Cu-TCNQ electric charge transfer complex and an alumina electrode is disclosed (See also, JP-A-2003-283004)
However, a switching device using the electric charge transfer complex has several problems. First of all, since the electric charge transfer complex is constituted of an electron-donating molecule and an electron-accepting molecule at a rate of 1:1 by using a co-deposition method or the like, it has a shortcoming such that it is difficult to control a composition thereof. Further, the switching device using the electric charge transfer complex has not sufficient reproducibility in the switching phenomenon and all devices produced in a same production condition do not necessarily exhibit switching characteristics. Namely, there is a problem such that appearance probability of devices which perform switching (transfer) is low. Further, even when the transfer is noticed, there is a problem such that, particularly, a transfer voltage from the off state to the on state is not constant.
Although a cause of such fluctuation as described above is not yet understood, reasons as described below are considered. Namely, in the above-described switching device, trace amounts of electric charges are injected in an organic layer in the off state. Then, the thus-injected electric charges are accumulated in an organic film or at an interface between the organic film and a metallic electrode and, thereafter, when the thus-accumulated electric charges reach a given threshold value, a transfer to the on state occurs; however, it is considered that, since such injection of the trace amounts of electric charges in the off state as described above is not constant, the fluctuation occurs.
Further, particularly, when the electric charge injection is unduly large, it is considered that there is a case in which the off state can not be maintained and a bistable property itself is not exhibited; on the other hand, when the electric charge injection is unduly small, it is considered that there is a case in which, since the electric charges are not accumulated, the transfer to the on state is not performed and, therefore, the bistable property is not exhibited.
In view of such conventional technical problems as described above, it would be desirable to provide a switching device in which an organic bistable material is disposed between electrodes and, then, appearance probability (transfer probability) of devices which perform switching is enhanced, fluctuation of a transfer voltage is small and switching characteristics are stabilized.